The present invention is generally related to control of electromechanical machines, and, more particularly, the present invention is related to method and system for controlling a permanent magnet (PM) machine without using high resolution absolute rotor position.
In the starting and control of permanent magnet machines using sinusoidal current control, the absolute position of the rotor of the machine is generally required to appropriately position the stator""s current vector relative to the rotor""s magnet flux vector in order for the machine to develop a desired level of torque.
It is known that various techniques have been proposed to determine absolute rotor position. Some of the proposed techniques require the use of resolver devices. Although resolver devices can accurately determine the absolute position of the rotor, such devices generally require tight positional tolerances and associated driving circuitry, and thus their relatively high cost incrementally adds to the overall cost of a propulsion system that uses permanent magnet machines.
Other techniques have attempted to determine the absolute position of the rotor without the use of sensors. Unfortunately, sensorless techniques may require computationally intensive algorithms and may not be suitable to applications, such as those using flywheel alternator starter systems, where a relatively high initial torque is desired so that, for example, an internal combustion engine coupled to the starter system can be started as quickly as possible under extreme environmental conditions.
Unfortunately, absolute position sensors based on standard position-pulse techniques have limitations since such sensors may not be effectively utilized for applications that require high initial starting torque since the absolute position is not determined until a pulse indicative of relative position of the rotor is received by a controller. For example, the rotor position determination may not occur for up to about one mechanical revolution of the rotor, during which the controller would be unable to appropriately position the current vector relative to the flux vector. This condition could lead either to reduced torque generation or even to negative torque generation.
In view of the foregoing issues, it is desirable to provide techniques capable of producing high initial starting torque without having to use a high resolution absolute position sensor. For example, the assignee of the present invention has proposed to use a low-cost and reliable sensing scheme that allows the machine to seamlessly transition from a brushless direct current (DC) mode of operation to an alternating current (AC) mode of operation. See attorney docket No. DP-304,528 for a detailed description of such scheme. The present invention proposes to take advantage of certain physical properties of PM machines. For example, it is known that torque production in a PM machine is due to two unique physical properties. The first property is based on current/flux interaction and the second property is based on reluctance principles. Thus, it will be appreciated that the torque characteristics of a PM machine will be a function of both current magnitude and phase relative to the position of the rotor of the machine. An example of this torque characteristic is shown in FIG. 1. It will be observed from FIG. 1 that the level of torque can vary significantly even over a relatively narrow angular range, e.g., a 60-degree range, of rotor position misalignment relative to the rotor position corresponding to the peak torque of the machine. It will be further observed from FIG. 1 that the peak torque value is relatively insensitive to small angular changes (e.g., +/xe2x88x9210 degrees) about the rotor position corresponding to that torque value.
The present invention proposes cost-efficient and reliable techniques that sense how the machine reacts to an initial estimate of rotor position. If for a given rotor position estimation the rotor turns, then that estimate of rotor position is likely to be sufficiently close relative to the actual rotor position corresponding to the peak torque of the machine. Conversely, if the rotor does not turn, then a processor could be used to perform an algorithm for incrementally adjusting the estimated rotor position through several other positions until sufficient torque is achieved to overcome the frictional and compressive forces of the internal combustion engine coupled to the starter system. It would be desired that such an algorithm be able to quickly start the machine while ensuring the ability to generate full torque. The level of the incremental adjustments of estimated rotor position can be customized, e.g., step wise, to the torque characteristics of the machine to optimize the foregoing objectives. That is, being able to quickly generate a high level of torque sufficient to start the machine in a desired direction. It would be further desirable to ensure that rotation of the rotor does not stall prior to processing a signal that allows to substantially align the estimated rotor position relative to the actual rotor position of the machine. In view of the foregoing discussion, it would be desirable to be able to start and control a PM machine without resorting to complex control algorithms, expensive sensors or without having to perform burdensome hardware modifications to the machine.
Generally, the present invention fulfills the foregoing needs by providing in one aspect thereof a method for controlling the start of a permanent magnet machine. The method allows to assign a parameter value indicative of an estimated initial rotor position of the machine. The method further allows to energize the machine with a level of current being sufficiently high to start rotor motion in a desired direction in the event the initial rotor position estimate is sufficiently close to the actual rotor position of the machine. A sensing action allows to sense whether any incremental changes in rotor position occurs in response to the energizing action. In the event no changes in rotor position are sensed, the method allows to incrementally adjust the estimated rotor position by a first set of angular values until changes in rotor position are sensed. In the event changes in rotor position are sensed, the method allows to provide a rotor alignment signal as rotor motion continues. The alignment signal allows to align the estimated rotor position relative to the actual rotor position.
The present invention further fulfills the foregoing needs by providing in another aspect thereof, a system for controlling the start of a permanent magnet machine. The system includes a parameter-assignment module configured to assign a parameter value indicative of an estimated initial rotor position of the machine. An energization module is configured to energize the machine with a level of current being sufficiently high to start rotor motion in a desired direction in the event the initial rotor position estimate is sufficiently close to the actual rotor position of the machine. A sensor is configured to sense whether any incremental changes in rotor position occurs in response to the energizing current. A processor is coupled to the incremental sensor to implement the following actions:
in the event no changes in rotor position are sensed, incrementally adjusting the estimated rotor position by a first set of angular values until changes in rotor position are sensed;
in the event changes in rotor position are sensed, processing a rotor alignment signal as rotor motion continues; and
aligning the estimated rotor position relative to the actual rotor position based on the alignment signal.